In a recording apparatus or the like using a disc or the like, its memory capacity increases and the transferring speed of the data becomes high in recent years. Therefore, a bearing device used in this kind of recording apparatus needs rotating function at high speed with high accuracy, and a hydrodynamic bearing device is used in its rotating main shaft portion.
Hereinafter, one example of a conventional hydrodynamic bearing device will be explained with reference to FIGS. 9 to 13.
As shown in FIGS. 9 and 10, a sleeve body 30 having a bearing hole 30a is constructed by sintered metal made by sintering metallic particles of copper alloy or the like, and is integrally inserted and fixed into an inside of a sleeve cover 31 which is made by working metal or resin. A sleeve is constructed by the sleeve body 30 and the sleeve cover 31. A shaft 32 is rotatably fitted in the bearing hole 30a of the sleeve body 30. The shaft 32 integrally has a thrust flange 33, the thrust flange 33 is housed in a space part 42 surrounded by the sleeve body 30, the sleeve cover 31 and the thrust plate 34, and the thrust flange 33 is rotatably provided in a posture sandwiched by the thrust plate 34 and the sleeve body 30 with both surfaces opposed to the thrust plate 34 and the sleeve body 30.
A rotor hub 35 is fixed to the shaft 32, and a rotor magnet 36 is fixed to an inner periphery of a large diameter part of the rotor hub 35. A motor stator 37 is mounted to a base 38 to oppose to the rotor magnet 36. Dynamic pressure generating grooves 39A and 39B are provided in at least one of an inner peripheral surface of the bearing hole 30a of the sleeve body 30 and an outer peripheral surface of the shaft 32 which opposes to this, and a dynamic pressure generating groove 40A is provided in opposed surfaces of the thrust flange 33 and the thrust plate 34. In accordance with necessity, a dynamic pressure generating groove 40B is provided in at least one of opposed surfaces of the thrust flange 33 and the sleeve body 30, and oil 41 as a working fluid is filled in a gap between the shaft 32 and the sleeve body 30 which the dynamic pressure generating grooves 39A, 39B, 40A and 40B face and a space part 42 in which the thrust flange 33 is placed.
An operation of the conventional hydrodynamic bearing device constructed as above will be explained by using FIGS. 9 and 10. In FIG. 9, when the motor stator 37 is energized, a rotating magnetic field occurs first, and the shaft 32, the thrust flange 33 and the rotor magnet 36 rotate integrally with the rotor hub 35. At this time, the dynamic pressure generating grooves 39A, 39B, 40A and 40B causes the oil 41 to generate pumping pressure, and the surface in which the dynamic pressure generating grooves 39A, 39B, 40A and 40B are formed floats with respect to the opposed surface and rotates without contacting the opposed surface. Namely, at the location of the dynamic pressure generating grooves 39A and 39B, a radial bearing for rotatably supporting in the state having a predetermined gap in a radial direction is formed, and at the locations of the dynamic pressure generating grooves 40A and 40B, a thrust bearing for rotatably supporting in a state having a predetermined gap in a thrust direction is formed.
According to the hydrodynamic bearing device of the above-described construction, by forming the sleeve body 30 of sintered metal, it is possible to put the sintered product into the mold and press-forming it, and therefore, the dynamic pressure generating grooves 39A, 39B, 40A and 40B can be manufactured with high accuracy while saving time and effort. As a result, it is not necessary to perform groove machining by precision cutting or the like in the tail end process as in the case where a sleeve body is manufactured by using an ordinary metal material, and the manufacturing cost can be reduced.
However, the hydrodynamic bearing device of the conventional construction as described above has the following problems.
As shown in FIG. 10, the shaft 32 is roatably inserted into the bearing hole 30a of the sleeve body 30, and the sleeve body 30 formed of sintered metal is made of a material having pores 30d of about 2 to 15% by area therein, including copper alloy of 60% by weight or more, and having the pores 30d impregnated with the oil 41 at low pressure. Accordingly, if there is rise in the temperature of the inside of the bearing or the like, the oil 41 with which the sleeve body 30 is impregnated is to go out of the sleeve body 30, and if the outer peripheral surface of the sleeve body 30 is exposed to the outside, the oil 41 sometimes flows out to the bearing outer portion and contaminates the ambient air.
FIG. 11 shows a surface picture of the sleeve body 30, and as shown in FIG. 11, the sleeve body 30 has the pores 30d in the surface. Therefore, the pressure of 2 atmospheric pressure to 5 atmospheric pressure, which occurs to the inside of the bearing by the actions of the dynamic pressure generating grooves 39A, 39B, 40A and 40B, leaks from the pores 30d of the surface by about 30%, which reduces rigidity of the radial bearing by 30%, and the shaft 32 sometimes cannot perform non-contact rotation, but contacts the sleeve body 30 during rotation and is rubbed. FIG. 12 shows the calculation value of the ratio (%) by which the radial bearing rigidity reduces in accordance with the area porousness rate (%) of the surface. FIG. 11 is the enlarged picture of the range of 0.3 mm×0.32 mm in the sintered metal surface, the black portions show the pores 30d and the white portions show the metal portion (with shine).
Accordingly, the hydrodynamic bearing device of the conventional construction needs the sleeve cover 31 to cover the sleeve body 30 from the outer periphery, and the sleeve cover 31 suppresses leakage of the oil 41 outside the bearing to the minimum.
In order to cope with such a problem, as the construction to inhibit leakage of the oil 41 outside the bearing, a hydrodynamic bearing device in which the surface of the sleeve body 30 fitted in the sleeve cover 31 is covered with the covering layer with impermeability for oil is disclosed in Patent document 1 (Japanese Patent Laid-Open No. 2003-322145) or the like.
According to this construction, the surface of the sleeve body 30 is covered with the covering layer with impermeability for oil, and therefore, oil does not enter the pores of the sleeve body 30, as a result of which, the oil 41 can be prevented from flowing outside the bearing through the sleeve body 30. Reduction in pressure of the oil 41 inside the bearing can be also prevented, and reduction in the radial bearing rigidity can be prevented.
However, the hydrodynamic bearing device with the above-described conventional construction has the following problems.
As shown in FIG. 13, when the sleeve body 30 which is inserted and fixed into the sleeve cover 31 is inclined and fixed, a deviation occurs to perpendicularity between the bearing hole 30a and the thrust plate 34, therefore causing the problems that stable support cannot be made with the gaps of the thrust bearing and the radial bearing become uneven, and the shaft 32 rubs against the inner surface of the sleeve body 30 and cannot make rotation without contact when the aforesaid deviation is large. Since the operation of inserting and fixing the sleeve body 30 into the sleeve cover 31 is needed and the sleeve body 30 and the sleeve cover 31 are separately provided, there is the disadvantage that the number of components increases correspondingly.
FIG. 14 shows the calculation values of the radial bearing radius gap (μm), viscosity of the oil 41 (mm2/s) and the bearing rotation friction torque (g/cm) in each temperature. The linear expansion coefficient of the copper alloy of the sleeve body 30 is 20.5×10−6 (/° C.) while the linear expansion coefficient of the martensitic stainless steel of the shaft 32 is 10.3×10−6 (/° C.), and therefore, there are the problems that the viscosity of the oil 41 becomes extremely large at low temperature while the inner diameter of the sleeve body 30 becomes so small that the radius gap between the bearing hole 30a and the shaft 32 becomes small, and thus the rotation becomes low. Accordingly, in the motor using this hydrodynamic bearing device, electric current consumption increases at the low temperature.